Mixed-cell-height detailed placement considering complex minimum-implant-area constraints

Abstract

Mixed-cell-height circuits have prevailed in advanced technology to address various design needs. Along with device scaling, complex minimum-implant-area (MIA) constraints arise as an emerging challenge in modern circuit designs, adding to the difficulties in mixed-cell-height placement. Existing MIA-aware detailed placement with single-row-height standard cells is insufficient for mixed-cell-height designs: (1) filler insertion, typically used to resolve MIA violations, might incur unaffordable area and wirelength overheads, and (2) mixed-height cell perturbation could cause severe inter-row MIA violations. This paper presents the first work to address the mixed-cell-height detailed placement problem considering both intra- and inter-row MIA constraints. We first fix intra-row violations by clustering violating mixed-height cells of the same threshold voltage, and then perturb each cluster to obtain a desired cell permutation by applying an efficient, optimal dynamic-programming-based algorithm for a special case and Algorithm DLX for general ones, where a provably constant performance ratio for a mixed-cell-height reshaping problem can be achieved. With a network-flow-based formulation, remaining violating cells are placed in appropriate filler-insertion positions to fix cell violations and minimize area. After performing mixed-cell-height detailed placement, we finally fix inter-row violations by shifting violating cells in minimum displacement. Compared with a filler insertion method and a greedy clustering approach, experimental results show that our proposed algorithm can resolve all MIA violations with smallest HPWL and area overheads in reasonable running time.